1. Field of the Invention
The present invention relates to a light source driving device and a related method, and more particularly, to a light source driving device capable of keeping constant current sink through dynamically detecting a dimming process and related method.
2. Description of the Prior Art
Light emitting diodes (LEDs) offer advantages of energy savings, long device lifetime, no mercury used, high achievable color gamut, without idle time, and fast response speed, so that LED technology is widely applied in fields of display and illumination. In addition, compared with a conventional light source device, light emitting diodes are suitable for fabrication as a tiny device or an array device, such as in traffic lights, outdoor displays, backlight modules of liquid crystal displays, PDAs, notebooks, or mobile phones with features of small size, shock resistance, ease of mass production, and high applicability.
Regarding physical properties of LEDs, current passing through an LED increases exponentially with increasing forward bias of the LED, and the brightness of the LED is directly proportional to the current through the LED. As more current passes through the LED, more brightness is available. Thus, in the prior art, an LED driving device usually uses a pulse width modulation (PWM) dimming mechanism to adjust the brightness of the LED. The PWM dimming mechanism takes advantage of a PWM signal to control an average current provided by a current source to the LED. When the PWM signal is at high logical level, the current source is conducted to provide a current for the LED. When the PWM signal is at low logic level, the current source stops providing the current for the LED. Therefore, if the PWM signal stays at the high logic level longer, the LED shines brighter. In other words, the brightness of the LED can be controlled by changing duty cycle of the PWM signal.
However, while performing the abovementioned PWM dimming process, since the current source and the LED are alternately connected to and disconnected from each other, the output driving voltage for driving the LED may exhibit a high ripple effect. As a result, a headroom voltage of the current source, i.e. available voltage value for the current source on each LED path, will change with the variation of the output driving voltage. In such a condition, the headroom voltage may be too high or too low, and will result in some unwanted effects. For example, if the headroom voltage is too high, the power consumption of the current source will increase, and the power conversion efficiency will be reduced. If the headroom voltage is not high enough, the current source will operate in an improper state, and cannot keep constant current sink, even to the point of not being able to provide the required driving current to the LED, and the LED will not conduct.